Camperdown Weeping Elm

(Question)

Leaf Miner and dead branches pruned out. Always used Cygon 2. Think no longer available. 2 methods add ring around trunk or my choice was TSP full around the drip line spaced out. 1 Tea spoon of Cygon for every inch of trunk diameter. Please adv ice what is available now to accomplish protect tree. Thanks Bob

(Answer)

Damage to elm trees by leafminers is quite widespread in Eastern Canada and once it gains a foothold it is difficult to control. I’m afraid to say there is no “quick fix”. Leafminer damage is unsightly but won’t really pose a significant threat to the health of your tree. It’s only when the infestation is severe that the tree will be weakened. However, leafminers can severely damage leafy vegetable crops. Leafminers are host specific – for elms the culprit is hymenoptera (which includes wasps, bees & sawflies)

Leaf miners feed on leaf tissue between the upper and lowers surfaces of a leaf. They leave highly visible trails that eventually turn brown and look unsightly. It’s important to understand the life cycle of the leafminer. The insects become active in the spring and lay their eggs soon after young leaves begin to grow on the tree. The eggs turn into larvae and it’s then that they start to feed on leaf tissue. In late spring the larvae themselves mature and eventually drop to the ground. The insect overwinters in the top soil, about an inch down and turn into a cocoon. They remain there and emerge in the spring and the cycle begins again.

There really isn’t a lot you can do. You probably know the use of pesticides in Ontario is now regulated and many products are banned. In the case of the leaf miner these insecticides had limited success anyway because the larvae are somewhat protected inside the leaf. If you have a small or young tree, infected leaves can be removed by hand. Tedious! However, with a well established tree this is not at all feasible. You can live with the damage and hope the insects will move on. There are natural enemies out there that normally regulate populations of these pests. You could try those sticky yellow traps, in early spring, to catch the flying insects before they lay their eggs.

There are some cultural practices that will help reduce the problem. Continue with the regular maintenance of your tree – watering, fertilizing, pruning and especially good sanitation practices. Prune out dead branches (which you have been doing) or branches with mined leaves. Make sure you clear up any plant debris under your tree – remove leaves and dispose of as this will help reduce overwintering populations of insects. Raked up leaves should be disposed of immediately. Do not use infected plant material for composting as this will perpetuate the problem.

If you wish to get the opinion of an arborist go to Landscape Ontario or the International Society of Arboriculture for help in choosing a professional in the field. Visit the Toronto Master Gardener website at www.torontomastergarders.ca and look for links under “Resources”.